Treatment of Wastewater Phosphate by Reductive Dissolution of Iron: Use of Ferric Oxyhydroxide Media
2011
Robertson, W. D. | Lombardo, P. S.
In smaller wastewater treatment systems such as septic systems, there is an interest in the development of passive phosphorus (P) removal methods. This study tested fixed-bed filters containing ferric oxyhydroxide media for wastewater P removal in a laboratory column test and in a full-scale domestic septic system. In the column test, during 30 mo of dosing with domestic wastewater, reductive iron dissolution reactions delivered consistent moderate concentrations of Fe into solution (2.9 ± 1.6 mg L⁻¹), and influent PO₄–P of 3.7 ± 1.0 mg L⁻¹ was attenuated to 0.09 + 0.04 mg L⁻¹ in the column effluent (98% removal). Phosphorus breakthrough at successive locations along the column indicated that in addition to sorption, mineral precipitation reactions probably also played an important role in the observed P attenuation. This was supported by electron microprobe analyses, which showed the presence of thick (20 μm) secondary Fe-rich coatings containing P on the primary ferric media grains. Assays of NaHCO₃–leachable and acid-extractable P on the column solids showed accumulation of up to 5.4 mg g⁻¹ acid-extractable P near the column inlet, but <5% of this amount was easily desorbable, further indicating P attenuation from processes other than sorption. Over 19 mo of operation, the domestic septic system also showed generally consistent increased Fe in the filter effluent (2.6 ± 1.7 mg L⁻¹) and achieved 99% P removal to 0.03 ± 0.02 mg L⁻¹ when the effluent was subsequently oxidized in a sand filter. Ferric iron filters could be attractive options for P removal in smaller wastewater systems because of their passive nature.
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